This invention relates generally to gas turbine engines, and, more particularly, to gas turbine engines which include low pressure turbine shrouds.
A gas turbine engine typically includes a combustor assembly, a high pressure turbine, and a low pressure turbine. In known engines, the combustor assembly and the high and low pressure turbines are disposed within an engine casing. A plurality of static shrouds abut together to form a ring shroud circumferentially disposed around the low pressure turbine within the engine casing. Each static shroud includes a backplate and abradable material attached to the backplate. The abradable material has various thickness extending across the backplate. Areas of increased thickness provide a seal to prevent hot combustion gases from flowing between adjoining static shrouds and impinging upon the engine casing. Furthermore, the areas of increased thickness extend axially between adjoining low pressure stator and rotor assemblies and maintain axial clearances between such rotor and stator assemblies.
During operation, air and fuel are mixed and ignited within the combustor. As hot gases exit the combustor and flow into the low pressure turbine, the static shrouds surrounding the low pressure turbine shield the engine casing from the hot gases. Because the thickness of the abradable material varies across the downstream side of the backplate, a thermal response rate across the backplate varies relative to the thickness of the abradable material. As a result, thermal gradients develop across the backplate and between the backplate and the abradable material.
Such gradients are prevalent during accelerations of the engine from idle power operations to take-off power operations. Over time, such thermal gradients induce thermal stresses and may contribute to low cycle fatigue of the shroud. The thermal stresses may weaken the backplate material leading to an eventual failure of the backplate. Such backplate failures permit the hot gases exiting the combustor to impinge upon previously shielded rotor assemblies, stator assemblies, and engine casing.
In an exemplary embodiment, a low pressure turbine shroud disposed within a gas turbine engine minimizes thermal gradients during engine operations. The shroud includes a backplate and abradable material attached to the backplate. The backplate includes an upstream end, a downstream end, and a body extending between the upstream and downstream ends. The body includes a first portion and a second portion. The first portion extends from the upstream end to the body second portion which extends to the downstream end. The body first portion has a first thickness and the body second portion has a second thickness. The abradable material attaches to an inner surface of the backplate and includes at least one segment having a thickness greater than the remaining segments of abradable material.
During engine operations, hot combustion gases enter the low pressure turbine and are shielded from an engine casing surrounding the low pressure turbine by the shroud. The areas of the backplate having an increased areas of thickness reduce a thermal response rate of the backplate such that the thermal response rate of the backplate body first portion is approximately equal that of a thermal response rate of the backplate body second portion. As a result, thermal gradients across the backplate are reduced.